1. Field of the Invention
The present invention relates to a laser plasma light source adapted to provide a light source powerful in an extremely short wavelength region particularly extending from the extreme ultraviolet region over the X-raywavelength region and to a method for generating radiation rays using the light source.
2. Discussion of the Background
As a small-sized light source having high brightness in the extreme ultraviolet region and X-ray region required in various fields including X-ray spectroscopy and X-ray measurement, laser plasma light sources that generate radiation rays by irradiating solid matter with pulsed laser beams have been used advantageously. However, laser plasma light sources pose a serious problem. It is a fact that a laser-plasma generates a large quantity of debris (scattered solid matter). In some cases, an optical element for collecting extreme ultraviolet rays or X-rays over a wide range of angles, such as a reflecting mirror, is used in order to effectively utilize a plasma light source that is a scattering light source. The optical element used in this case is prone to damage and contamination in the presence of debris.
Such laser plasma light sources are nevertheless attracting attention in the development of X-ray reduction lithography, which promises to become the lithography of the next century. In order to enable practical application to this technology, however, extremely strict suppression of debris generation has to be ensured so that the decrease in reflectance of a multilayer reflecting mirror can be reduced to not more than several percent even after a continuous operation of not less than 109 shots. To be specific, when the pulse energy of laser beams is 1 J and the efficiency of conversion into X-rays is 1%, the total quantity of debris attached to an optical surface is required to be not more than about 109 pg/sr/shot
Various attempts have been made to suppress debris. For example, the inventor et al. carried out quantitative evaluation of debris [T. Tomie et al., Proc. SPIE 831 (1987) 224]. In addition, it has been reported that the quantity of debris reaching a position 10 cm from the plasma was reduced by approximately two orders in an experiment conducted for generating a plasma in a He gas. Furthermore, it was also reported that the quantity of debris can be reduced by making a solid target thinner. However, the quantity of debris after reduction by these means was larger by several orders than that strictly required in reduction lithography.
On the other hand, Mochizuki et al. proposed the idea of using a cryo-target made of a rare gas, such as Xe, solidified, that could avoid the problem of the quantity of debris because deposition probability of rare gases to optical surfaces would be low [Mochizuki et al., Proc. SPIE 773 (1987) 246]. G. D. Kubiak et al. experimented on this idea at the Sandia National Laboratory, California, U.S.A. and developed a system of pellezing a solidified rare gas and impelling the pellets into a position to be irradiated with pulsed laser beams [G. D. Kubiak et al., Tech. Digest Extreme Ultraviolet Lithography (Monterey, 1994 Sep.) Tu D1 pp. 82-84].
In addition, a liquid drop target was proposed in Lund University at the City of Lund, Sweden as means for solving the engineering problem of continuous supply of targets in the rare gas pellet system. An alcohol was jetted out of a high-speed vibrating nozzle to produce alcohol drops 10 .mu.m in diameter at 1 MHz repetition ratet. When these drops were irradiated with laser beams having a pulse width of 70 ps, pulse energy of 70 mJ and a wavelength of 0.5 .mu.m, the efficiency of conversion into X-rays having a wavelength of 3 nm was around 1%. An experiment was conducted for generating long-wavelength X-rays required in reduction lithography, using laser beams having a pulse width of 8 ns, pulse energy of 700 mJ and a wavelength of 1 .mu.m, and the efficiency of conversion into X-rays having a wavelength of 13 nm was around 0.1% [L. Mahnqvist et al., OSA TOPS on Extreme Ultraviolet Lithography, 1996, Vol. 4, eds. G. D. Kubiak and D. R. Kania (Opt. Soc. Am., Washington, D.C., 1996) pp. 72-74]. The total quantity of debris was estimated to be 6 pg/sr/pulse from the quantity of the debris attached to a glass plate disposed in the vicinity of the target [L. Rymell and H. M. Hertz, Rev. Sci. Instrum. 66 (1995) 4916]. However, since alcohol drops are composed of oxygen and carbon that have a high conversion efficiency in a wavelength region of 2 to 3 nm and are elements not effective for generating X-rays in any other wavelength region.
A system of jetting Xe gas from a jet nozzle was developed at the Sandia National Laboratory and reported [G. D. Kubiak et al., OSA TOPS on Extreme Ultraviolet Lithography, 1996, Vol. 4, eds. G. D. Kubiak and D. R. Kania (Opt. Soc. Am., Washington, D. C., 1996) pp. 66-71]. This report indicates that the ratio of the rare gas attached to and deposited on the optical surface is extremely low, that the substances attached to a multilayer are only the materials for the jet nozzle and its cooling yoke, and that the quantity of the rare gas attached to the optical surface is as small as 17 pg/shot.
According to the liquid drop system and the gas jetting system mentioned above, the total quantity of debris estimated from the quantity of the debris attached to the optical surface can be reduced by three orders in comparison with the system using a solid target, whereas the efficiency of conversion of x-rays is merely one half, i.e. 0.5%, that obtained with a solid target of gold. The conversion efficiency is desired to be around several times from the standpoints of realization of a practical system and lightening the burden of excitation laser beams.
Depending on what the light sources of this lind are used for, radiation rays with various wavelengths have to be generated. On the other hand, since the wavelength peak and bandwidth of X-rays generated from a plasma vary depending on the elements, various elements are to be converted to a plasma. In the gas jetting system, however, the materials used are limited to Xe, Ar, N.sub.2 and the like. The wavelength of the X-rays from the Xe gas jetting light source used at the Sandia National Laboratory has been found to be 11.5 nm, deviating from the 13 mn at which a Mo/Si multilayer easiest to fabricate is utilizable. Therefore, development of a novel multilayer utilizable at that X-ray wavelength is urgently needed. In the liquid drop system used at Lund University, the elements used are limited to oxygen, carbon, nitrogen and the like.
Further, in reduction lithography, one of the main issues is how to make the wave front aberration in an optical system small, and it is considered that adoption of a ring field illumination using a region of small wave front aberration is indispensable. For the ring field illumination, a method of arcuately sweeping the position of an X-ray source at high speed using the deflection of laser beams is used advantageously. In the gas jetting system or liquid drop system, however, high-speed sweeping ofthejetting position is simultaneously required. This is not easy.
According to the liquid drop system or gas jetting system, the quantity of debris attaching to an optical surface can be greatly reduced, whereas the amount of liquid to be removed or gas to be vaporized and removed increases. In the liquid drop system, assuming that irradiation can be operated at 1 KHz, only 0.1% of a drop train produced at 1 MHz repetition rate can be utilized. For this reason, it will be necessary to adopt some means for recovering the liquid drops without their being vaporized in order to lighten the burden of the discharge system. In the rare gas jetting system, the time duration of jetting gas from a nozzle required for producing a jetting region of hundreds of .mu.m is 1 .mu.s at most while the shutter speed of an ordinary plasma electromagnetic valve is not less than 100 .mu.s. This means that the amount of gas jetted is excessive by not less than 100 times that required. Therefore, the rare gas jetting system poses a gas-evacuation speed problem, rather than the debris-generating problem, and can be consequently regarded as a system that cannot solve any substantial problem. The various methods proposed heretofore are excellent only from the aspect of suppression of the debris quantity estimated from the quantity of the debris attached to an optical surface, but are dissatisfactory when totally considering the aspects of conversion efficiency, wavelength variability, X-ray source position sweeping property and other significant factors and cannot be put to practical use.